STEM technique maps all the atoms in any molecule

Transmission electron microscopy (TEM) is a technique that should, in theory,
facilitate direct imaging and chemical identification of each and every atom in
a material that has an unspecified three-dimensional structure. This is especially
true given the recent introduction of aberration-corrected optics. Until recently,
however, neither TEM nor any other method has proved up to the task in nonperiodic
materials.

Now, however, a group of researchers has devised a technique for
imaging and identifying atoms in almost any solid or molecule. The team recently
used its creation to resolve and directly identify every atom in a monolayer of
boron nitride, a feat never before performed.

The researchers, who included those affiliated with Nion Co. of
Kirkland, Wash., Vanderbilt University in Nashville, Oak Ridge National Laboratory
and the University of Oxford in the UK, reported in the March 25, 2010, issue of
Nature that they had used annular dark-field imaging in an aberration-corrected
scanning transmission electron microscope (STEM) in their study.

Using aberration-corrected annular
dark-field electron microscopy (left), researchers for the first time have located
and identified all atoms in a nonperiodic sample of boron nitride with substitutional
impurities. Shown is an experimentally determined model superimposed on the image,
with boron (red) and nitrogen (green), along with impurities carbon (yellow) and
oxygen (blue). Courtesy of Ondrej L. Krivanek, Nion Co.
This type of atom-by-atom analysis could be performed on organic
and other molecules that do not crystallize into an ordered array, said Ondrej L.
Krivanek, president of Nion Co. and lead author.

“With the 1-Å – or 100-pm – resolution
we now roughly have for light atoms, all the atoms should be identifiable, except
probably hydrogen, and the atomic structure of any general molecule should therefore
be analyzable in principle,” said Krivanek.

Because electrons are one-fiftieth the size of an atom, they have
long been considered prime candidates for direct atomic imaging and identification.
However, this has been done only for crystals, which have a regular structure. That
has now changed, thanks to various enabling technologies.

One such technology is aberration correction of electron beam
imaging, an area that Krivanek said Nion pioneered and in which it is a leader.
This has improved imaging resolution by a factor of three. Another advance was the
development of very stable electron beams, achieved by eliminating all sources of
noise. The result is that the beam used in the current experiments is still to 5
pm rms – 10 times better than is typical.

Other enabling technologies were the aforementioned annular dark-field
detection, as well as cold-field emission and ultrahigh-vacuum processing. The first technique,
which recently was extended to light atoms, allows chemical identification because
the signal is strongly dependent upon atomic number. The second one optimizes resolution
when using a lower-energy electron beam, a necessity because these beams minimize
radiation damage to structures with light atoms such as boron, carbon, nitrogen
and oxygen. Ultrahigh vacuum around the sample prevented stray contaminants from
being picked up during the analysis.

Imaging in the study was performed using a Z-contrast UltraSTEM
scanning transmission electron microscope made by Nion, with a corrector of third-
and fifth-order aberrations and a cold-field emission electron source, which helped
to minimize beam blurring.

The aberration corrector and the cold-field electron source gave
the researchers excellent resolution at 60 kV. This was important, according to
Stephen J. Pennycook, a materials science and technology researcher at Oak Ridge,
because operating at this low voltage allowed them to avoid atom displacement damage
to the sample.

The use of Z-contrast STEM was integral to the experiment. “The
Z-contrast mode is the only way to distinguish the elements based on their intensity
in the image,” Pennycook said. Phase contrast imaging is the other common
high-resolution TEM mode. Here, however, the atoms are very close in intensity and
impossible to distinguish one after the other. Z-contrast imaging uses electrons
scattered through larger angles, scattering off the nucleus or Rutherford scattering,
and thus is much more sensitive to the species of atom.

“The other big advantage of a Z-contrast microscope,”
Pennycook added, “is that it also allows electron energy loss spectroscopy,
when transmitted electrons are analyzed for their loss of energy. Although this
is a lower-level signal, it is element-specific, so we knew for certain that there
was a lot of carbon on the specimen.”

After building and tuning the microscope, the researchers examined
boron nitride monolayers. They did this for two reasons, Krivanek explained. One
is as a proof of principle of the elemental analysis technique. The second was that
boron nitride sheets are potentially useful because the material is related to graphene,
which could form the basis for future electronics. Devices could someday be built
out of graphene, with nonconducting boron nitride separators.

In their study, the researchers corrected the aberration in an
electron beam, swept it across the boron nitride monolayer sample and collected
the resulting dark-field image. This revealed individual atoms.

By analyzing the molecular structure of experimental materials,
atom by atom, researchers can identify structural defects in those materials. This
is significant because defects, including the presence of an impurity atom or molecule,
often determine the material’s properties.

They also found atomic substitutions in the monolayer. For example,
there was carbon in the place of boron at some locations, carbon in the place of
nitrogen at others and oxygen in the place of nitrogen elsewhere. These atoms, which
differed in size from the ones they replaced, caused distortions in the monolayer
of about one-tenth of an angstrom, or about 10 pm. These observations were in agreement
with calculated values.

Although this proves that the technique works, Krivanek noted
that there are limitations to keep in mind. The most important is that major radiation
damage can occur in a sample, arising from the impact of the electrons in the beam.
There are ways around this, particularly if many identical copies of a molecule
are present. As that is often the case in biology, the approach could be applicable
to the study of organic molecules.

The researchers plan to explore the electron energy loss spectroscopy
signal in more detail and hope to correlate the defects observed with electronic
changes in the material. This also is very important for graphene, a potential replacement
for silicon (which is nearing the famous “end-of-the-road map,” Pennycook
said, so next-generation devices will have to use a different material). Graphene
offers the right characteristics, but developers have not yet come up with a reliable
means to make repeatable devices with it.

“The key is to have the right impurities in the right places
and not the wrong ones,” Pennycook said. “So seeing and identifying
the impurity atoms is the key to understanding electrical properties and, ultimately,
to new devices.”

As for the future, Krivanek said of the technique, “We can
now see individual atoms more clearly than we ever could before. Since the entire
world is made of atoms, the potential range of applications is very wide indeed.”